1. Field of the Invention
The present invention relates to a linear voltage regulator with a wide input voltage range.
2. Description of the Background Art
Linear voltage regulators produce a regulated output voltage from an input voltage. For voltage regulation, a differential or operational amplifier can be used, for example, whose non-inverting input is supplied with a constant reference voltage and whose inverting input is connected to a tap of a voltage divider, which is connected between a terminal for an output voltage and a reference voltage, typically ground. An output of the differential or operational amplifier is connected to what is called a pass transistor, which is connected between a terminal for the input voltage and the terminal for the output voltage. The pass transistor is driven as a function of the voltage difference at the differential or operational amplifier and changes its forward resistance accordingly, by which means the desired, regulated output voltage is established.
Proper function of such linear voltage regulators generally requires the input voltage to be greater than the desired output voltage by a defined minimum amount, since a voltage drop takes place at the pass transistor, with the input voltage and the output voltage differing by the amount of this voltage drop.
Regardless of this circumstance, in operating conditions in which the input voltage is too small, and in particular smaller than the desired output voltage, it is possible to produce an output voltage that is much too small, or even no output voltage at all. A cause of this can be, for example, that in spite of a decrease in the output voltage, the voltage difference at the differential amplifier does not increase such that the pass transistor is switched on sufficiently. This results in an excessive voltage drop at the pass transistor, and thus an output voltage that is too small.